One common weighing apparatus is a weighing scale. Various weighing scales (or scales), such as those for weighing vehicles or other objects, typically include a load receiving element, conveyor belt, or other load receiving element for receiving an object to be weighed. The load receiving element is normally supported by a plurality of subjacent load cells, which load cells may have an analog or digital output. The weight of an object residing on the load receiving element exerts a force of some magnitude on each load cell. The magnitude of the force imparted to a load cell corresponds to the portion of the weight of the object supported by the given load cell.
In weighing scales employing multiple analog load cells, the load cells are typically electrically connected in parallel to form a combined circuit and the output of that combined circuit is, in effect, the average of the load cell outputs. The outputs are connected to a secondary device (e.g., a meter) whose function is to convert the combined circuit output into a weight value. Because metrology requirements dictate that a scale must provide weights that are independent of object position on the load receiving element, the sensitivities of the load cells must be adjusted to be substantially equal. To accomplish this, the output of more sensitive load cells is adjusted to that of less sensitive load cells, either by lowering the excitation voltage supplied to more sensitive load cells or by adding an isolated resistor across their outputs. The resultant circuit output then becomes representative of the output of the least sensitive load cell.
As described above, the load cells of a weighing scale employing multiple analog load cells are typically electrically connected in parallel. Consequently, and as would be well understood by one of skill in the art, the overall sensitivity of such a weighing scale is reduced as the total number of load cells is increased.
In contrast to the above-described analog construction, a weighing scale may employ digital load cells, meaning that each load cell produces a numerical value as output, the numerical value indicative of the force imparted thereto by the presence of an object on the load receiving element. One embodiment of such a digital load cell is described in U.S. Pat. No. 4,815,547. In a weighing scale employing digital load cells, the output of each load cell is corrected to compensate for different locations of a load on the load receiving element. To this end, a multiplicative correction factor may be determined for each load cell in order to provide a scale output that is independent of object position on the load receiving element. Larger factors are applied to load cells that are less sensitive and smaller factors are applied to load cells that are more sensitive. Load cell correction factors can determined in any number of ways, such as by the technique described in U.S. Pat. No. 4,804,052. The scale output of such a digital system is determined by numerically summing the outputs of all the load cells of the scale.
Digital load cells need not be connected in parallel, as is typically the case in a weighing scale using a plurality of analog load cells. Rather, the weight of a load placed on a weighing scale employing digital load cells is determined by numerically summing the outputs of the individual digital load cells, as opposed to converting the output of a combined circuit of analog load cells into a weight value. As such, in contrast to a weighing scale employing a number of analog load cells, the sensitivity of a weighing scale employing a number of digital load cells does not decrease as the number of load cells is increased.
In the past, it has been common practice to manufacture both analog and digital type weighing scales utilizing load cells of like capacity. Correction of load cell output once assembled to the scale structure and exposed to a load is then accomplished as described above, depending on whether the scale uses analog or digital load cells.
It has been determined, however, that the load cells of a multiple-load cell scale often do not support an equal portion of the weight of a given object to be weighed. Rather, and as will be explained and illustrated in more detail below, one or a group of load cells typically supports more of the object weight than another load cell or another group of load cells.
It would be understood by one of skill in the art that known and typical scale design would require the selection of load cells having a like sensitivity and, therefore, a capacity sufficient to accommodate the highest loading likely to be seen by the associated scale. However, in considering the above-described disproportionate weight distribution among the load cells of a multiple-load cell scale, it can also be understood that this is not a desirable situation. Particularly, even though certain load cells of such a typical scale will likely be exposed to less force than other load cells, their capacity will nonetheless be the same as those load cells that will be exposed to greater forces. Consequently, these former load cells will have excess capacity and such a design may result in a scale of reduced sensitivity.
As would be obvious, load cells of greater capacity generally cost more than load cells of less capacity due to their larger size, more robust construction, etc. Consequently, at least in the case of a weighing scale constructed with digital load cells (or possibly, analog load cells connected to one or more digital conversion boxes), using load cells of lesser capacity where possible can reduce the overall cost of the scale. To that end, there is a cost benefit to constructing a digital load cell weighing scale in this manner. As is discussed in more detail below, there may also be a metrology benefit.